In a typical mild hybrid vehicle, an internal combustion engine provides the power necessary for propelling the vehicle, with the engine being configured to shut off when the vehicle is idle or at a standstill. In this manner, fuel may be conserved, particularly during stop-and-go traffic conditions. When a driver depresses an accelerator pedal to launch a mild hybrid vehicle, an electric drive motor connected to a 12-volt auxiliary battery provides an initial burst of power lasting through a duration of time required for cranking and starting the engine, which is approximately 400 to 500 milliseconds (ms). The drive motor used in such a mild hybrid design is not used to power the vehicle independently of the engine, as would a conventional or “full” hybrid vehicle. However, mild hybrid vehicles remain desirable for some purposes, as such vehicles may be configured to provide, for example, regenerative braking and/or idle stop capabilities while reducing required engine size.
The 12-volt auxiliary battery provides the necessary direct-current (DC) voltage and associated DC current needed for cranking the engine, and also provides a sufficient auxiliary DC voltage for use by various vehicle systems, for example headlights, interior lights, and wiper blade systems. However, because of the relatively high electrical load placed on the battery during cranking of the engine, a temporary or transient reduction in the amount of voltage supplied to the auxiliary system may occur. This reduction in voltage, referred to as “voltage sag” hereinafter, typically lasts through the same 400 to 500 ms duration of time required for cranking and starting of the engine discussed above. If this voltage sag exceeds a threshold level, the result may become perceptible to an operator or passenger of the vehicle. For example, the headlights or interior lights may dim momentarily, and/or windshield wiper speed may temporarily decrease or pause. While a dedicated secondary battery may provide a sufficient amount of cranking assist voltage to allow the auxiliary battery to supply a substantially constant voltage to the auxiliary systems, a duplicate battery may be less than optimal due to its added size, weight, and cost.